1. Field of the Invention
The present invention relates to an arrangement for utilizing the heat contained in the exhaust gas or combustion gas of a coal-fired boiler which forms a component of a power plant, for preheating feed water for the boiler and for preheating primary air. The arrangement includes a rotating heat transfer unit to which are admitted, on the one hand, the exhaust gas leaving the boiler and, on the other hand, the preheated combustion air, and a heat shifting unit connected in parallel to the heat transfer unit, wherein primary air, on the one hand, and cooled exhaust gas discharged from the heat transfer unit, on the other hand, can be admitted to the heat shifting unit.
2. Description of the Related Art
An arrangement of the above-described type is known from the brochure of GEA Lufitkuhler GmbH, Dorstener Str. 18-29, 4690 Herne 2, Federal Republic of Germany, "Kraftwerk: Rauchgaswarme fur die Vorheizung von Luft und Kondensat" [Power Plant: Flue Gas Heat for Preheating Air and Condensate]. In this arrangement, the heat carrier medium of the heat transfer unit, which flows in closed circulation, is conducted through a heat exchanger to which primary air, whose temperature corresponds to that of the ambient air, and through a heat exchanger to which is admitted another heat carrier medium which also flows in closed circulation and which is heated by the exhaust gas discharged by the heat transfer unit. This additional heat carrier medium also flows through a heat exchanger, to the other side of which is admitted the boiler feed water.
Although the exhaust gas emerging from the boiler which is fired with brown coal has a temperature of approximately 290.degree. C., the exhaust gas available at the heat shifting unit has a temperature of only approximately 180.degree. C., because of the heat exchange taking place in the rotating heat transfer unit. Accordingly, in view of the heat losses in the heat shifting unit, on the one hand, and in the circulation of the heat transfer medium between the heat shifting unit and the boiler feed water, on the other hand, the boiler feed water can only be heated to a maximum temperature of approximately 160.degree. C. This means that a substantial amount of fuel must be fired in the boiler in order to further heat the boiler feed water which had been heated to approximately 160.degree. C., to vaporize the water and, subsequently, to feed the steam to a turbine for producing electrical energy.